1. Field of the Invention
The present invention relates to a waveguide photodetector and, more specifically, to a waveguide photodetector that can efficiently couple with an optical fiber or planar lightwave circuit (PLC) and operate even at high input power.
2. Discussion of Related Art
In general, a photodetector converts optical signals into electric signals. This photodetector is typically formed of semiconductor materials, and different semiconductor materials are used for different purposes.
When the photodetector is formed of a semiconductor material, a semiconductor absorbing layer absorbs light to generate electrons and holes. The generated electrons and holes are transported to respective electrodes, thus producing electric signals.
For example, when a PIN diode photodetector is reverse biased, generated electrons are transported to an n-electrode, and generated holes are transported to a p-electrode. The operating speed of a PIN diode depends on a transit time, which refers to a time taken for generated charges to move to respective electrodes, and an RC effect, which is related with the capacitance and load resistance of the PIN diode.
That is, in order to operate the PIN diode at high speed, the transit time should be shortened and the capacitance of the PIN diode should be reduced. The transit time can be shortened by reducing the thickness of an absorbing layer, but a reduction in the thickness of the absorbing layer leads to an increase in the capacitance of the PIN diode. Accordingly, the absorbing layer should be formed to an appropriate thickness according to its purpose.
Further, considering efficient integration with an optical device such as a planar lightwave circuit (PLC), a waveguide photodetector should be adopted. This waveguide photodetector can operate faster than a conventional surface-illumination photodetector. Because of these advantages, side-illumination waveguide photodetectors have widely been developed.
The efficiency of the waveguide photodetector is significantly affected by its capability of efficiently coupling with an optical fiber or PLC. In general, the beam size of light produced from the optical fiber or PLC is about 7 to 10 μm in diameter. On the other hand, it is difficult to obtain a semiconductor waveguide that can efficiently guide light having such a large beam size of about 7 to 10 μm.
To overcome this drawback, a multimode waveguide photodetector is disclosed in “A High Efficiency 50 GHz InGaAs Multimode Waveguide Photodetector” [IEEE J. of Quantum Electronics, vol. 28, No. 12, pp. 2728–2735, 1992].
In the multimode waveguide photodetector, incident light is not confined in only an InGaAs absorbing layer but also spreads over a p-InGaAsP layer and an n-InGaAsP layer, which constitute a separate confinement heterostructure (SCH) on both sides of the absorbing layer. Thus, the multimode waveguide photodetector can detect light while guiding light having a large spot size.
However, since the InGaAs absorbing layer has a great thickness, a large amount of incident light is confined in the InGaAs absorbing layer. Thus, when high-power light is absorbed in the InGaAs absorbing layer, excessive electrons and holes are generated. As a result, a voltage applied to the photodetector drops, thereby slowing down the operating speed of the photodetector. In other words, when high-power light is incident on the InGaAs absorbing layer, the operating speed of the photodetector is reduced.
Meanwhile, a waveguide photodetector, which operates at high speed even at high power, is disclosed in U.S. Pat. No. 6,278,820 B1. In the waveguide photodetector, a core layer is disposed under an absorbing layer. Also, a large amount of light is guided by the core layer, while a slight amount of light is incident on the absorbing layer. Thus, even if high-power light is incident on the photodetector, the absorbing layer generates electrons and holes in small amounts.
As a result, the photodetector disclosed in U.S. Pat. No. 6,278,820 B1 can prevent the slowdown of the photodetector at high power and can guide light having a great spot size only by slight etching.
However, when the photodetector disclosed in U.S. Pat. No. 6,278,820 B1 combines with a PLC, the absorbing layer and an intermediate layer constitute an integral waveguide together with the core layer of a main waveguide. As a result, the central axis of a waveguide mode deviates from the center of the core layer. Therefore, it is difficult to exactly find the central axis of the waveguide mode.